The present application relates generally to sterilization of objects, and more particularly to sterilization of medical apparatus using both hydrogen peroxide vapor and a glow discharge.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Some background information can be found in the following documents, all of which are hereby incorporated by reference: Beatriz Unger-Bimczok, Volker Kottke, Christian Hertel, Johannes Rauschnabel, “The Influence of Humidity, Hydrogen Peroxide Concentration, and Condensation on the Inactivation of Geobacillus stearothermophilus Spores with Hydrogen Peroxide Vapor”, Journal of Pharmaceutical Innovation, Vol. 3, No. 2 (28 Jun. 2008), pp. 123-133; James R. Rickloff “Factors Influencing Hydrogen Peroxide Gas Sterilant Efficacy”, Advanced Barrier Inc. Nov. 12, 2008; U.S. Pat. Nos. 4,169,123, 4,169,124, 4,643,876, 4,756,882, 4,956,145, 4,642,165, and 4,744,951; PCT application WO 2005/067984; the Sterrad NX Sterilizer user and service manuals (from Advance Sterilization Products); and the Sterrad 100S Sterilizer user manual and service manuals.
Medical instruments were traditionally sterilized either with heat, such as is provided by steam, or a chemical, such as formaldehyde or ethylene oxide in the gas or vapor state. Each of these methods has drawbacks. Many medical devices, such as fiberoptic devices, endoscopes, power tools, etc. are sensitive to heat, moisture, or both. Formaldehyde and ethylene oxide are both toxic gases that pose a potential hazard to healthcare workers. Problems with ethylene oxide are particularly severe, because its use requires long aeration times to remove the gas from articles that have been sterilized. This makes the sterilization cycle time undesirably long.
Sterilization using liquid hydrogen peroxide solution has been found to require high concentration of sterilant, extended exposure time and/or elevated temperatures. However, sterilization using hydrogen peroxide vapor has been shown to have some advantages over other chemical sterilization processes (see, e.g., the '123 and '124 documents cited above).
The combination of hydrogen peroxide with a plasma provides certain additional advantages, as disclosed in the '876 document cited above. The '882 document cited above discloses the use of hydrogen peroxide vapor, generated from an aqueous solution of hydrogen peroxide, as a precursor of the reactive species generated by a plasma generator. The combination of hydrogen peroxide vapor diffusing into close proximity with the article to be sterilized and plasma acts to sterilize the articles, even within closed packages.
However, these methods of combining hydrogen peroxide vapor with a plasma, while useful in “open” systems, have been found to be inadequate to effect sterilization in articles having diffusion-restricted areas, since the methods are dependent upon diffusion of the sterilant vapor into close proximity with the article before sterilization can be achieved. Thus, in order to use these methods on articles with long, narrow lumens, it has been necessary to use high concentration of sterilant, extended exposure time, and/or elevated temperatures. For example, lumens longer than 40 cm and/or having an internal diameter of less than 0.4 cm have been particularly difficult to sterilize. Thus, no simple, safe, effective method of sterilizing longer and smaller lumens exists in the prior art.
The sterilization of articles containing diffusion-restricted areas, such as long narrow lumens, presents a special challenge for hydrogen peroxide vapor that has been generated from an aqueous solution of hydrogen peroxide, because: (i) water (H2O) has a higher vapor pressure than hydrogen peroxide (H2O2), and will vaporize faster than hydrogen peroxide from an aqueous solution; (ii) water has a lower molecular weight than hydrogen peroxide and will diffuse faster than hydrogen peroxide in the vapor state.
Because of this, when an aqueous solution of hydrogen peroxide is vaporized, the innermost locations in a diffusion-restricted lumen will initially see an enhanced H2O:H2O2 ratio. This can lead to condensation of water vapor on the surface of the material to be sterilized before sufficient impingement of hydrogen peroxide has reached the innermost locations. The liquid-phase water then becomes a barrier to the penetration of hydrogen peroxide vapor into diffusion-restricted areas, such as small crevices and long narrow lumens.
The '145 document cited above discusses the efficacy of highly concentrated hydrogen peroxide for the safe sterilization. The '067984 document discusses the problem of condensed water vapor blocking the diffusion of the sterilant to the bacteria lying on the surface of the material to be sterilized. The Unger document cited above explains the influence of humidity, hydrogen peroxide concentration, and the condensation of the water vapor in detail.
One cannot solve the problem by using more concentrated hydrogen peroxide, since concentrated solutions of hydrogen peroxide, i.e., greater than 60% by weight, can be hazardous, due to the oxidizing nature of the solution. Decomposition of liquid hydrogen peroxide is very exothermic, and releases large volumes of gas, so that stability is a serious concern. Highly-concentrated liquid hydrogen peroxide is so energetic that it has been used as a monopropellant for rocket engines. Moreover, highly concentrated hydrogen peroxide can form unstable reaction products with minor contaminants (such as fingerprint grease), and those reaction products can be a further source of instability.
The above-cited documents '165 (Bier) and '951 (Cummings et al.) both attempt to address this problem. Bier attempts to solve the problem by metering small increments of a hydrogen peroxide solution onto a heated surface to ensure that each increment is vaporized before the next increment is added. This helps to eliminate the difference in the vapor pressure and volatility between hydrogen peroxide and water, but it does not address the fact that water diffuses faster than hydrogen peroxide in the vapor state.
Cummings describes a process for concentrating hydrogen peroxide from a relatively dilute solution of hydrogen peroxide and water and supplying the concentrated hydrogen peroxide in vapor form to a sterilization chamber. The process involves vaporizing a major portion of the water from the solution and removing the water vapor produced before injecting the concentrated hydrogen peroxide vapor into the sterilization chamber as shown in FIG. 1A.
FIG. 1A shows the apparatus proposed by Cummings, which includes a vaporizing chamber 7 having any well-known means 3 for injecting into chamber 7 a predetermined amount of a solution of hydrogen peroxide and water. Chamber 7 may be controllably heated by any well-known means. Chamber 7 has an outlet port 2 through which vapors may be exhausted from chamber 7 by means of a vacuum. Port 2 may be opened or closed by valve 11. Chamber 7 also has an outlet port 14 leading through passage 6 to a sterilization chamber 8. Passage 6 may be open or closed by valve 5.
When valve 5 is closed and valve 1 is open; vacuum is applied to chamber 7 to evacuate air. Chamber 7 is heated until the desired temperature within chamber 7 is reached; that temperature is such that, when taken with the pressure within chamber 7, water in the form of vapor will be flashed from a solution of liquid hydrogen peroxide and water present in chamber 7. The process is initiated by the injection into evacuated chamber 7 of predetermined amount of a liquid solution of hydrogen peroxide and water through injection means 3. Conditions within chamber 7 cause the preferential vaporization of water from the solution and the vapor formed thereby is withdrawn from chamber 7 through port 2. At a point in time when a major portion of the water in the injected solution has been vaporized and withdrawn, but before a significant quantity of hydrogen peroxide has vaporized and been withdrawn, valve 1 is closed. What remains in chamber 7 is a hydrogen peroxide-water solution enriched in hydrogen peroxide, specifically greater than 40% hydrogen peroxide by weight, preferably 50 to 80% by weight. Vaporization of this enriched solution continues within chamber 7 and then valve 5 is opened to admit the vapors formed thereby to evacuated sterilization chamber 8. With a substantial amount of the water having been removed, the hydrogen peroxide vapor sterilant is able to disperse itself throughout the sterilizer and penetrate wraps and tubes without encountering a barrier effect that otherwise would have been present by reason of the effects of the present of water discussed above. Thus, the effective concentration of hydrogen peroxide vapor at the point of attack on the goods to be sterilized is markedly enhanced by the process.
Advance Sterilization Products, a division of Johnson and Johnson, introduced The Sterrad NX Sterilizer which employs Cummings method of delivering hydrogen peroxide to sterilize devices within the sterilization chamber. In this apparatus a 59% aqueous solution of hydrogen peroxide is injected into the delivery system condenser where it is condensed and concentrated and then introduced into the chamber. This modified process concentrates the 59% hydrogen peroxide to 90% nominal hydrogen peroxide (by selectively vaporizing and removing water) prior to being transferred into the sterilization chamber.